1. Field of the Invention
The present invention relates to a photovoltaic element having a specific doped layer. More particularly, the present invention relates to a photovoltaic element having a semiconductor junction comprising a specific doped layer containing a plurality of regions having a diminished density of an element belonging to group IV of the periodic table (this element will be hereinafter referred to as group IV element) as a principal constituent of said doped layer such that they are intermittently distributed in the inplane of said doped layer and which exhibits improved photovoltaic element characteristics.
The foregoing region having a diminished density of the group IV element will be hereinafter referred to as "group IV element low density region".
2. Related Background Art
There are presently a variety of photovoltaic elements (solar cells) which have been put into practical use as photoelectric conversion devices for converting light such as sunlight into an electric energy. Particularly, photovoltaic elements frequently have been used for power supply sources in commercial and home appliances. Besides, they sometimes have been used as power supply sources in vehicles and also in houses.
Incidentally, photovoltaic elements are expected to be a future power generation source which can substitute for oil-fired power generation and nuclear power generation, because they provides clean energy without causing CO.sub.2 buildup as in the case of the oil-fired power generation and without causing radioactive wastes and radioactive materials as in the case of the nuclear power generation. In view of this, various studies have been conducted of photovoltaic elements.
These photovoltaic elements (solar cells) include single crystal photovoltaic elements, polycrystal photovoltaic elements, amorphous photovoltaic elements, copper indium selenide photovoltaic elements, and compound semiconductor photovoltaic elements.
Photovoltaic elements (solar cells) are based on the technology of utilizing photoelectromotive force generated in a semiconductor active region having a pn junction. In the semiconductor active region, light such as sunlight is absorbed, the absorbed light generates photocarriers including electrons and holes, and the photocarriers are drifted by the action of an internal electric field of the pn junction, whereby photoelectromotive force is outputted to the outside as an electric power.
Of the presently known photovoltaic elements, a photovoltaic element made of a single crystalline silicon material has been found to be highly reliable and high in photoelectric conversion efficiency (this photovoltaic element will be hereinafter called "single crystal silicon photovoltaic element"). However, there are disadvantages for the single crystal silicon photovoltaic element as will be described in the following. That is, it is unavoidably costly since it is produced by way of the so-called semiconductor wafer process wherein a single crystalline silicon material whose valence electron having been controlled to p- or n-type is prepared by way of crystal growth by means of CZ process, the resultant single crystalline material is sliced to obtain a silicon wafer of about 300 .mu.m in thickness, and the resultant silicon wafer is made have a layer with a conduction type which is opposite to that of the silicon wafer, for instance, by a manner of diffusing a given valence electron-controlling agent, whereby a semiconductor active region having a pn junction is formed. In addition, the single crystalline silicon material by which the single crystal silicon photovoltaic element is constituted is relatively small in light absorbance because of indirect transition and therefore, it is necessary for the single crystal silicon photovoltaic element to have a thick semiconductor active region with a thickness of at least 50 .mu.m in order to facilitate its function to absorb light such as sunlight. Further, the single crystalline silicon material by which the single crystal silicon photovoltaic element is of about 1.1 eV in band gap and because of this, short wavelength energy components of the sunlight spectrum are not efficiently utilized for photoelectric conversion in the single crystal silicon photovoltaic element. Further in addition, it is extremely difficult for the single crystal silicon photovoltaic element to be of a large area because there is a limit for the size of silicon wafers that can be produced because of the requirement for growing a single crystal. Therefore, in order to obtain a large quantity of electric power, it is necessary that a number of single crystal silicon photovoltaic elements are integrated in series or parallel connection by way of wiring. Besides, in the case where the single crystal silicon photovoltaic element is used in outdoors, it is necessary to have expensive mounting in order to protect the single crystal silicon photovoltaic element from suffering from damages caused by various meteorological conditions. In this connection, the cost of the generated energy per unit is rather expensive in comparison with that of existing power generation system.
There are known photovoltaic elements made of a polycrystalline silicon material (this photovoltaic element will be hereinafter called "polycrystal silicon photovoltaic element"). The polycrystal silicon photovoltaic element is advantageous in that it can be produced at a cost which is lower than that of the single crystal silicon photovoltaic element. However, for the polycrystal silicon photovoltaic element, there are disadvantages similar to those in the case of the single crystal silicon photovoltaic element. Particularly, the polycrystalline silicon material by which the polycrystal silicon photovoltaic element is constituted is relatively low in light absorbance because of indirect transition and therefore, it is necessary for the polycrystal silicon photovoltaic element to have a thick semiconductor active region as well as in the case of the single crystal silicon photovoltaic element. In addition, the semiconductor active region of the polycrystal silicon photovoltaic element contains grain boundaries and because of this, the polycrystal silicon photovoltaic element is not satisfactory in terms of photovoltaic element characteristics.
In view of the above, photovoltaic elements made of a non-single crystalline material, for instance, such as an amorphous material (this photovoltaic element will be hereinafter referred to as non-single crystal photovoltaic element) have been spotlighted because of advantages such that they can be relatively easily formed in a large area at a reasonable cost. Some of these non-single crystal photovoltaic elements have been put into practical use as power supply sources installed in outdoors. And various studies have been made in order to achieve a non-single crystal photovoltaic having an improved power-generating performance.
As such non-single crystal photovoltaic element, there can be mentioned, for example, amorphous photovoltaic elements whose photovoltaic force-generating layer being constituted by an amorphous semiconductor material of a tetrahedral group IV element such as amorphous silicon (a-Si), amorphous silicon-germanium (a-SiGe) or amorphous silicon carbide (a-SiC); and compound semiconductor photovoltaic elements whose photovoltaic force-generating layer being constituted by a compound semiconductor material comprising groups II and VI elements such as CdS or Cu.sub.2 S or a compound semiconductor material comprising groups III and V elements such as GaAs or GaAlAs. Of these, so-called thin film amorphous photovoltaic elements whose photovoltaic force-generating layer being constituted by such amorphous semiconductor material as above described have various advantages which are not provided in the case of a single crystal photovoltaic element, such that their constituent semiconductor film can be relatively easily formed in a large area, thinned as desired, and formed on an appropriate substrate.
There is, however, a disadvantage of such an amorphous photovoltaic element in that its photoelectric conversion efficiency is inferior to that of the single crystal photovoltaic element. In this connection, various studies have been made in order to attain a reliable amorphous photovoltaic element having a improved photoelectric conversion efficiency which can be desirably used a daily power supply source.
There are various proposals for improving the photoelectric conversion efficiency of the non-single crystal photovoltaic element. For instance, in order for a non-single crystal photovoltaic element having a pin semiconductor junction to have an improved photoelectric conversion efficiency, there is known a manner of improving the property of each of the constituent p-type semiconductor layer, i-type semiconductor layer, n-type semiconductor layer, transparent electrode layer, and back face electrode layer.
U.S. Pat. No. 2,949,498 proposes a technique of attaining an improved photoelectric conversion efficiency by employing a stacked structure comprising a plurality of photovoltaic cell units having a pn semiconductor junction being stacked. This technique can be applied in not only in the case of an amorphous photovoltaic element but also in the case of a crystalline photovoltaic element. In accordance with this technique, it is possible that the sunlight spectrum is efficiently absorbed by plural photovoltaic cell units each having a different band gap to increase the open-circuit voltage (Voc) whereby attaining an improved power generation efficiency. This technique using the stacked cell structure is to improve the photoelectric conversion efficiency by stacking a plurality of photovoltaic cell units each having a different band gap so that the respective energy components of the sunlight spectrum can be absorbed by said photovoltaic cell units. In this case, the so-called top layer in each photovoltaic cell unit situated on the light incident side is made have a band gap which is larger than that of the so-called bottom layer situated under the top layer.
However, also in the case of this technique using stacked cell structure as well as in the case of using a single cell structure, it is necessary to improve the property of each of the constituent p-type semiconductor layer, i-type semiconductor layer, n-type semiconductor layer, transparent electrode layer, and back face electrode layer, in order to attain a desirable photoelectric conversion efficiency. For instance, the i-type semiconductor layer is necessary to be designed such that it has an appropriate band gap depending upon the photovoltaic force-generating layer structure such as single cell structure or stacked cell structure. In addition, it is necessary to decrease the localized states as much as possible and to heighten the mobility of photocarrier.
Besides the above proposal to improve the property of the i-type semiconductor layer in order to attain an improvement in the photoelectric conversion efficiency of a photovoltaic element, there are other proposals. For instance, U.S. Pat. No. 4,254,429 and U.S. Pat. No. 4,377,723 propose a technique of using a buffer layer of providing a graded band gap at the junction interface of the i-type semiconductor layer with the p-type semiconductor layer or/and the n-type semiconductor layer. It is understood that the buffer layer herein is used for the following purpose. Since at the junction interface of the i-type semiconductor layer constituted by an a-SiGe material with the p-type or n-type semiconductor layer constituted by an a-Si material, interface states are generated due to differences in the lattice constant, such states are prevented from generating by using a given a-Si material as the buffer layer at the junction interface to make photocarrier effectively mobilize whereby attaining an improved open-circuit voltage (Voc).
There is also a proposal of providing a so-called graded layer in the i-type semiconductor layer constituted by an a-SiGe material by changing the composition ratio between the silicon atoms and germanium atoms of a part of the a-SiGe material as the constituent of the i-type semiconductor layer to improve the characteristics of the i-type semiconductor layer. Particularly, for instance, U.S. Pat. No. 4,816,082 discloses a technique in that in the constitution comprising an i-type semiconductor layer interposed between first and second semiconductor layers whose valence electron having been controlled, the i-type semiconductor layer is made have such a graded band gap that it is relatively large at the position in contact with the first semiconductor layer situated on the light incident side, followed by gradually decreasing toward the central position, then followed by gradually increasing toward the second semiconductor layer. According to this technique, carriers generated from light impinged are efficiently separated by virtue of the internal electric field to improve the photoelectric conversion efficiency.
There is a further proposal of improving the hole mobility of the i-type semiconductor layer constituted by an a-Si or a-SiGe material by incorporating a slight amount of a valence electron-controlling agent of p-type into the i-type semiconductor layer because the i-type layer often has a slight n-type property.
By the way, for the so-called doped layer such as p-type semiconductor layer or n-type semiconductor layer, it is required that the density of an activated acceptor or donor is high and the activation energy of a thin film as the doped layer is small. By this, there is provided a large diffusion potential (built-in potential) when a pin semiconductor junction is formed, whereby the open-circuit voltage (Voc) of the photovoltaic element is increased, resulting in an improvement in the photoelectric conversion efficiency.
The doped layer is also required not to hinder light from impinging into the i-type semiconductor layer (which functions to generate photocurrent) to the utmost since the doped layer basically does not function to generate photocurrent. In this respect, it is important for the doped layer to have a large optical band gap and a thin thickness.
For the doped layer, it is further required that it has a property capable of forming a homo or hetero pin semiconductor junction with the i-type semiconductor layer and the interface states in the semiconductor junction are slight.
In the case of a stacked cell structure, the doped layer is required to have such characteristics as will be described below in addition to the above requirements. In the case where a plurality of cells having a pn or pin semiconductor junction are stacked, a reverse semiconductor junction portion where the p-type semiconductor layer and the n-type semiconductor layer are contacted is caused. This portion is required to establish a tunneling junction having an ohmic property and to be small in series resistance.
In order to attain such a doped layer having the foregoing property, there are known materials usable as the constituent of the doped layer and methods for forming the doped layer.
Specific examples of such constituent material usable as the doped layer are Si, SiC, SiN, and SiO. Specific examples of such method for forming the doped layer are RF plasma CVD, ECR plasma CVD, and photo CVD.
Particularly, as the constituent material of the doped layer situated on the back side of the i-type semiconductor layer with respect to the light incident side, amorphous silicon (a-Si) materials are preferable in view of easiness in layer formation. As the constituent material of the doped layer situated on the light incident side of the i-type semiconductor, amorphous silicon carbide (a-SiC) materials which are small in light absorption coefficient, and microcrystalline silicon (.mu.c-Si) materials which are small in light absorption coefficient and small in activation energy are preferable.
Now, in the case where the doped layer is constituted by, for instance, amorphous silicon carbide (a-SiC) material as a principal constituent which has been made to have a relatively large optical band gap by incorporating an appropriate element into said a-SiC material, by using, as said element, an element selected from the group consisting of H, C, N, O, and halogen elements (X) which are capable of enlarging the optical band gap and increasing the amount of the element to be added, it is possible to decrease the light absorption coefficient of the doped layer whereby increasing the short-circuit current (Jsc) of the photovoltaic element. However, for any of the foregoing elements capable of enlarging the optical band gap, even when the amount thereof added is increased, the activation energy of the doped layer tends to increase where the built-in potential of the photovoltaic element is decreased, resulting in a reduction in the open-circuit voltage (Voc). And there is a tendency that as the difference between the chemical composition of the doped layer and that of the i-type semiconductor layer is increased, the interface states at the interface between the doped layer and the i-type semiconductor layer are increased.
In addition, for the doped layer, as the concentration of a valence electron-controlling agent (a dopant) contained in the doped layer is increased, the activation energy of the doped layer is decreased where the built-in potential of the photovoltaic element is increased, resulting in an increase in the open-circuit voltage (Voc). However, there is a tendency that as the concentration of the dopant contained in the doped layer is increased, the optical band gap of the doped layer is decreased to increase the light absorption coefficient of the doped layer where the short-circuit current (Jsc) of the photovoltaic element is decreased.
With respect to the constituent of the doped layer, it is considered that the doped layer is preferred to be constituted by a microcrystalline material rather than an amorphous material, since in comparison of the former with the latter, the former has a smaller light absorption coefficient, a larger optical band gap and a smaller activation energy. However, in the case where the doped layer is constituted by the microcrystalline material, there are such problems as will be described in the following. It is difficult to form a desirable microcrystalline material in which the density of an activated acceptor or donor is high and which has a small activation energy.
Particularly, there is no report for a photovoltaic element having a relatively large open-circuit voltage (Voc) and a relatively large photoelectric conversion efficiency prepared using other microcrystalline materials than microcrystalline Si (.mu.c-Si). And there is found no example for practical use of those microcrystalline materials other than .mu.c-Si in the preparation of a photovoltaic element. Even in the case where a .mu.c-Si material is used, the .mu.c-Si material can be formed only under certain limited conditions which are difficult to be controlled.
Further, in the case where a microcrystalline doped layer is formed on an amorphous i-type semiconductor layer, since the conditions for the formation of the microcrystalline doped layer are different from those for the formation of the amorphous i-type semiconductor layer, the amorphous i-type semiconductor layer is liable to damage upon the formation of the microcrystalline doped layer. And in the case where the amorphous i-type semiconductor layer and the microcrystalline doped layer form a hetero semiconductor junction, there is a tendency that the interface states are increased to provide an adverse effect to a pin semiconductor junction when it is formed.
Further in addition, for the thickness of the doped layer in a photovoltaic element, regardless of the kind of a constituent material by which the doped layer is constituted, as the thickness is decreased, the quantity of light transmitted through the doped layer into the i-type semiconductor layer is increased where the short-circuit current (Jsc) of the photovoltaic element is increased. However, there is a tendency that as the thickness of the doped layer is decreased, the activation energy of the doped layer is increased where the built-in potential of the photovoltaic element is lowered, resulting in a reduction in the open-circuit voltage (Voc). In this connection, a optimum thickness of the doped layer which maximizes the photoelectric conversion efficiency of the photovoltaic element is within a limited small extent. When the doped layer has a thickness which is not within said extent, the photoelectric conversion of the photovoltaic element tends to decrease.
As above described, in the prior art, it is difficult to realize a desirable doped layer (a desirable p-type or n-type semiconductor layer) composed of a non-single crystalline material which has a large built-in potential and is slight in the interfacial level for use in a photovoltaic element. Hence, in order to attain a non-single crystal photovoltaic element having a more improved photoelectric conversion efficiency, it is important that in addition to more improving the i-type semiconductor layer, the doped layer is idealized.
Separately, in order to more improve the photoelectric conversion efficiency of the conventional photovoltaic element, various studies have been made. For instance, Miyazi et als. have reported a single cell type amorphous silicon photovoltaic element comprising a stacked reflection preventive film/glass/carbon graded p-type layer/buffer layer/i-type layer/n-type layer/ITO/Ag and which has a initial photoelectric conversion efficiency of 13.19% (see, the pamphlet of the 53th Applied Physics Society Conference in 1992, page 746, 17p-B-5).
J. Yang et al. has reported that using a profiled band gap amorphous silicon-germanium alloy in the bottom cell, a stabilized active-area efficiency of 11.16% has been achieved (see, Applied Physics Letters, 61(24), 1992, pp. 2917-2919).
However, according to any of the techniques described in these two reports, it is difficult to make the light-induced degradation of the initial photoelectric conversion efficiency fall in the range of less than 5%.
J. Meier et al. have reported a photovoltaic element in which a microcrystalline silicon (.mu.c-Si) material is used as the i-type semiconductor layer and which is slight in the light degradation and which has an initial photoelectric conversion efficiency of 9.1% (see, 1994 IEEE First World Conference on Photovoltaic Energy Conversion, pp. 409-412).
Referring to this report, it is understood that the photovoltaic element in which the .mu.c-Si material is used can be produced by a plasma CVD process by way of glow discharge as well as in the case of producing a conventional amorphous series photovoltaic element. And it is considered that a large area photovoltaic element having an i-type layer composed of a .mu.c-Si material could be produced by means of a relatively low-priced fabrication apparatus as well as in the case of producing an amorphous photovoltaic element having a large area. However, the photoelectric conversion efficiency of this microcrystalline photovoltaic element is inferior to that (about 13%) of the amorphous silicon single cell type photovoltaic element. Hence, for the photovoltaic element whose i-type semiconductor layer being composed of the .mu.c-Si material, there is an important subject to be solved in that at least the photoelectric conversion efficiency is improved.